Methods of preparing rutile seeding agents



Patented Dec. 39, 1947 UNETED STATES PATENT OFFER METHODS OF PREPARINGRUTILE. SEEDING AGENTS No Drawing. Application January 31;, 1942, SerialNo. 428,998

3 Claims. 1

This invention relates to the preparation of rutile titanium dioxide bythe calcination of an anatase precipitate under controlled conditions,and aims to provide a special method of making a seedtherefor.

Titanium dioxide, within the thirty years since its introduction as apigment, has made such rapid strides that it has become the outstandingwhite pigment used in the coating and allied industries; this popularityis due mainly to its hitenes a l y. h hiding p w h ghe tinting strength,and ready dispersibility in vehicles. It is prepared largely by thehydrolysis, under carefully controlled conditions, of titanium sulfatesolutions containing iron, prepared by digestion of ilmenite (a nativeferrous titanate) with sulfuric acid, followed by reduction of allferric iron to ferrous iron, removal of apart of the iron bycrystallization as ferrous sulfate, clarification and adjustment ofconcentration; the hydrolysate is calcined, together with conditioningagents, to produce the desired white pigment.

The titanium dioxide so produced has been pre: dominantly of anatasecrystal structure, one of the three crystal modifications (anatase,brookite, rutile) in which titanium dioxide occurs. The art has knownthat the rutile structure, on account of its higher refractive index,should have higher hiding power and tinting strength than the anatasestructure, but rutile titanium dioxide having better hiding power andtinting strength has not been available commercially be? cause there hasbeen no process for its production at a cost comparable with that ofproducing anatase.

It is well known that the uncalcined precipie tate obtained by thehydrolysis of titanium sulfate has the crystal structure of anatasewhich is further developed upon calcination as the combined water andthe combined and/or adsorbed acid are expelled. It is also known thatthe uncalcined precipitate obtained by the hydrolysis of titanium saltsof monobasic acids such as, for example, titanium chloride, has thecrystal structure of rutile which is further developed upon calcination. It is also known that the anatase structure obtained by thehydrolysis of titanium sulfate can be converted to rutile by continuedcalcination at a very high temperature. Such calcination, however,causes crystal growth and discoloration, which results in a productundesirable for use as a white pigment. No process in which the titaniumis precipitated from salts of monobasic acids is economicallycompetitive with the sulfate process largely because of higher acid costand the corrosion problems introduced by the use of these monobasicacids.

In co-pending patent application Serial No. 422,104, filed December 8,1941, now abandoned,

5 it is proposed to convert an anatase precipitate,

' obtained by the hydrolysis of titanium sulfate solutions, to rutile,by first roasting the precipitate in the presence of an alkali metalsalt, washing the roasted precipitate substantially free of 1Q alkalimetal salt, and then calcining in the presence of a small amount of arutile seed consisting of calcined titanium dioxide, having the crystalstructure of rutile, or consisting of an uncalcined precipitate obtainedby the hydrolysis of an aquee ous solution of a monobasic acid compoundof titanium. It is disclosed that the rutile seed may be added eitherprior to or after the roasting operation.

In co-pending patent application Serial No.

422,082, filed December 8, 1941, now abandoned, it is proposed toconvert an anatase precipitate, obtained by the hydrolysis of titaniumsulfate solutions, to rutile by calcining in the presence of a smallamount of an alkali metal compound and a small amount of a rutile seedconsisting of calcined titanium dioxide having the crystal structure ofrutile or consisting of an uncalcined precipitate obtained by hydrolysisof an aqueous solution of a monobasic acid compound of tita nium.

In co-pending Ross and Tanner application, Serial No. 429,118, filedJanuary 31, 1942, it is proposed to prepare a rutile seed for use in theproduction of titanium dioxide of rutile crystal structure by treatingan alkali metal titanate with limited quantities of a monobasic acid,such as hydrochloric acid. The titanate is treated with hydrochloricacid in sufficient quantity to combine with the sodium oxide and insufiicient excess for about 20% to about 50% of theory for the formationof titanium tetrachloride. About 25% of the theoretical quantity fortitanium tetrachloride after neutralizing the sodium oxide is preferred.The hydrochloric acid slurry is diluted withwater and boiled for aboutone hour. Nitric acid or other monobasic acids may be used in place ofhydrochloric acid. The amount of acid used is insufiicient for completesolution of titanium. The titanium, however, during the boiling period,is transferred to a desirable rutile seed for use in converting anatasestructure to rutile structure during calcination. Some such seedsprepared by us have shown by X-ray anal ysis substantially completerutile structure.

In that aboveeidentified application, thealkali 3 metal titanate isprepared from a sulfate hydrolysate which is boiled for several hours toobtain precipitation.

We have now discovered an especially economical method for thepreparation of such hydrated. titanium dioxide, in which a precipitatereadily reactable with alkali to form a titanate is ob- Our methodconsists tained in a few minutes.

in adding a titanium sulfate solution to a hot aqueous alkaline solutionin such relative proportions that the basicity of the mixed solution isincreased so that the molar ratio of available S03 to T102 is about 1.4to 1.0, corresponding, on a molar basis, to a factor of acidity of 40%,as a result of which substantially immediate precipiabout 50% of theoryfor the formation of titanium tation occurs. Substantially any sulfatesolution I or alkali can be used.

Example 1 The precipitation may be made, for example, from a clarifiedbasic sulfate solution of ilmenite ore, from which part of the iron hasbeen removed by crystallization as ferrous sulfate. A typical analysisof such a solution is as follows:

Specific gravity 1.42 Titanium dioxide (TiOz) per cent Ferrous sulfate(FeSOi) do 10 Sulfuric acid (H2504) combined with titanium dioxide percent 18 To 3 liters of boiling water is added 10 grams of hydrated lime,and then during about 10 minutes is added one liter of the basic sulfatesolution containing 150 grams of titanium dioxide at a temperature ofabout 60 C. The mixture is boiled for about five minutes, at the end ofwhich time about 90% of the titanium is precipitated. The precipitate isfiltered and washed substantially free of iron and calcium salts.

In our procedure for the conversion of hydrated titanium dioxide such asis made by the method of Example 1, to rutile seed, the hydrated oxideis treated with an alkaline alkali metal compound, such as sodiumhydroxide, potassium hydroxide, etc. The hydrated oxide is treated withfrom about one part to about five parts by weight of sodium hydroxide toone part of anhydrous titanium dioxide and heated for from about onehour to about six hours at a temperature of about 80 C. to 100 C. Aboutone and one-half parts of sodium hydroxide is a preferred amount. Solidflake sodium hydroxide may be mixed with the concentrated aqueous pulpof the hydrated titanium dioxide, and then heated without furtheragitation. A concentrated aqueous solution of sodium hydroxide may bemixed with the hydrated titanium dioxide pulp and heated whilecontinuously stirring. Depending on the amount of sodium hydroxide used,and also upon the concentration of the sodium hydroxide and hydratedtitanium dioxide mixture, the temperature may vary considerably up toand above 100 0., even up to the fusion point, if desired. The reactionmay be carried out under atmospheric conditions, or under pressure in anautoclave; under the latter conditions, the reaction mass remains fluid.To the sodium titanate thus formed, water, which causes some hydrolysisof the titanate, and consequent formation of hydrated titanium dioxideand/ or other titanate or titanates, is added. The solids are washed bydecantation and then filtered and washed. The washed solids contain, byanalysis after calcination, about 85% titanium dioxide and about sodiumoxide. No attempt is made herein toascribe any definite compositiontetrachloride. About 25% of the theoretical quantity for titaniumtetrachloride after neutralizing the sodium oxide is preferred. Thehydrochloric acid slurry is diluted with water and boiled for about onehour. Nitric acid or other monobasic acids may be used in place ofhydrochloric acid. The amount of acid used is ordinarily insufficientfor complete solution of titanium, although peptization generallyoccurs. The titanium, however, during the boiling period, is transformedto a desirable rutile seed for use in converting anatase structure torutile structure during calcination. Some such seeds prepared by us haveshown by X-ray analysis substantially complete rutile structure.

If desired, the titanate may be treated directly with the monobasic acidthus omitting the water treatment. We prefer to use the water treatment,however, since it eliminates a large amount of the alkali metal andsulfate, and thereby reduces the amount of monobasic acid required.Furthermore, a more active seed is generally obtained if the alkalimetal and sulfate are largely removed prior to the monobasic acidtreatment.

The rutile seed thus formed may be used as such by mixing with anuncalcined hydrated titanium dioxide precipitate, obtained by hydrolyticprecipitation from a titanium sulfate solution, at any stage in theprocess prior to the final washing before calcination, or it may befiocculated by adjusting the slurry to a pH of from about 4.5 to 7.5 byneutralizing with an alkaline reagent such as sodium carbonate,filtered, washed substantially free of chlorides and then mixed with anuncalcined hydrated titanium dioxide precipitate, obtained by hydrolyticprecipitation from a titanium sulfate solution at any stage in theprocess prior to calcination.

Our special rutile seed is extremely finely divided, which probablyaccounts for its exceptional activity. By its use as a seed even inrelatively small amounts, substantially complete conversion to rutilemay be obtained under ordinary calcination conditions.

The amount of rutile seed used may vary within wide limits. While ingeneral we prefer to 11:8 about 5% based upon the total weight of thetitanium dioxide calcined, smaller and larger percentages may be used.Even as little as 1% or less is effective. Obviously there is no upperlimit.

The actual amount of rutile seed used depends upon the amount of rutileconversion desired and also upon the desired other properties of thefinished pigment, such as color and softness. The use of 5% or more seedinduces conversion to rutile at a lower temperature or in a shortertime, or both, and thereby produces a pigment of rutile structure, whichhas a color higher in total brightness and which disperses more readilywhen ground in vehicles.

It is desired to place no limit upon the rutile calcined solids content.

conversion obtained, For certain purposes relatively small conversionmay be desired and in other cases a substantially complete conversionmay be desired. For example, it may be desirable to produce a pigmentcontaining 15% rutile and 85% anatase or a pigment containing 90% rutileand anatase. The hiding power and tinting strength of the pigmentproduced according to our process increases proportionately as theconversion to rutile increases.

A small amount of an iron salt, such as ferric ammonium sulfate, or asmall amount of copper salt, such as copper sulfate, which saltsdecompose and form the oxides of the metals upon calcination, may beadded at some stage prior to calcining to prevent bluing or graying ofthe color of the pigment during calcination. The preferred amount ofiron is from about .01% to about .03%, calculated as F6203, and basedupon the Weight of the titanium dioxide. If the uncalcined hydratedtitanium dioxide already contains an appreciable amount of iron, theamount added is correspondingly less than the .01% to .03%, so that thecalcined pigment will contain from .01% to .03% FezOs. The preferredamount of copper is from .0004% to .002%, calculated as CuO.

, Detailed methods for the preparation of the rutile seed are given inExamples 2 and 3.

Example 2 The precipitate of Example 1 (containing 100 grams TiOz) isdiluted with Water to about 30% To this aqueous slurry is added 150grams of fiake sodium hydroxide while stirring continuously. The mixtureis then heated for about two hours at a temperature of about 85 C. to 90C., While continuously stirring and ata constant volume. The titanatethus formed is diluted to about 1.5 liters with water, washed twice bydecantation, then filtered and washed substantially free of sulfates.The filter cake is slurried in 185 cc. of commercial 20 B. hydrochloricacid, diluted with one liter of water, and boiled at constant volume forabout one hour. The seed slurry is cooled to 60 C., and adjusted to a pHof 6.0 by the addition of about 340 cc. of a 20% sodium carbonatesolution. At this pH the seed is sufiiciently fiocculated to befiltered. It is filtered and washed substantially free of sodium andchloride ions.

Example 3 The precipitate of Example 1 (containing 100 grams of TiOz) isdiluted with water to about 30% calcined solids content. To this aqueousslurry is added 150 grams of flake sodium hydroxide while stirringcontinuously. The mixture is then heated for about two hours at atemperature of about 85 C. to 90 C., while stirring continuously and ata constant volume. The titanate thus formed is diluted to about 1 literswith water, washed twice by decantation, then filtered and washedsubstantially free of sulfates. The filter cake is slurried in 95 cc. ofcommercial 42 B. nitric acid, diluted with one liter of water, andboiled at constant volume for about one hour.

Having described our invention and given examples illustrating methodsfor preparing the rutile seed, we now give examples illustrating theconversion of anatase structure to rutile structure.

Example 4 A thoroughly washed aqueous pulp of an hydrated titaniumdioxide precipitate, obtained. by h-ydrolytic precipitation from atitanium sulfate solution, and containing 200 grams. of titaniumdioxide, is mixed with a quantity of the neutralized and washed seed ofExample 2 containing 6 grams of titanium dioxide, with 0.24 gram offerric ammonium sulfate containing .04 gram of ferric. oxide and with aconcentrated aqueous solution containing 4.1 grams of potassiumcarbonate. The mixture is dried and roasted for one hour at atemperature of about 850 C. The roasted: product is milled and slurriedin suilicient water to give a pulp containing 15% solids. 5 cc. ofcommercial 20 B. hydrochloric acid are added, and the solids arefiltered out and washed substantially free of sulfates. The washedfilter cake is then dried, and calcined by slowly raising thetemperature from about 300 C. to about 975 C. during about five hours,and then holding at a temperature of about 950 C. to about 1000 C. forabout two hours.

The transformation to rutile, as determined by X-ray analysis, has beensubstantially complete. The tinting strength of the hydroclassified anddry milled pigment is 1720.

Tinting strength evaluations are based upon an arbitrary scale in whichpresent commercial titanium dioxide of anatase crystal structure has avalue of 1250.

Example 5 A thoroughly washed aqueous pulp of an hydrated titaniumdioxide precipitate, obtained by hydrolytic precipitation from atitanium sulfate solution, and containing 200 grams of titanium dioxide,is mixed with a quantity of the neutralized, filtered and washed seed ofExample 2, containing 10 grams of titanium dioxide, and with aconcentrated aqueous solution containing 0.42 gram of potassiumcarbonate and calcined for about two hours at a temperature of about 975C. to about 1000 C.

The calcined pigment contains by X-ray analysis about rutile. Thetinting strength of the hydroclassified and dry milled pigment is 1700.The color of the product is not quite as good as that of the productobtained in Example 4, due to the omission of the iron addition.

Example 6 A thoroughly washed aqueous pulp of an hydrated titaniumdioxide precipitate, obtained by hydrolytic precipitation from atitanium sulfate solution and containing 200 grams of titanium dioxide,is diluted with water to a calcined solids content of 20% and mixed withgrams of the the nitric acid suspension of Example 3 containing 10 gramsof titanium dioxide. The mixture is filtered and the filter cake iswashed substantially free of nitrates. The filter cake is then mixedwith 0.24 gram of ferric ammonium sulfate containing .04 gram of ferricoxide, and with a concentrated aqueous solution containing 0.42 gram ofpotassium carbonate, dried and calcined by slowly raising thetemperature from about 300 C. to about 975 C. during about five hours,and then holding at a temperature of about 975 C. to about 1000 C. forabout three hours.

The transformation to rutile, as determined by X-ray analysis, has beensubstantially complete. The tinting strength of the hydroclassifiedpigment is 1720.

While a rather definite calcination schedule is indicated in theexamples, this schedule may be varied considerably. In actual commercialoper- 7 ation the calcination may be continuous through a rotatinghorizontal kiln which is heated at the discharge end and in whichtherefore there is a gradual temperature change from one end of the kilnto the other. a

Our calcined pigment, obtained in accordance with any of themodifications of the invention, may be either simply dry milled or wetmilled, hydroclassified, treated with the usual reagents, filtered,dried and disintegrated in accordance with the present known proceduresfor producing the commercial titanium dioxide pigments having theanatase crystal structure.

Our finished pigment has many desirable properties, such as goodresistance to chalking, excellent resistance to discoloration in whitebaking enamels, etc, but is characterized in the main by its exceptionalhiding power and tinting strength. The hiding power and tinting strengthof the product in which the conversion to rutile has been substantiallycomplete is in the neighborhood of 40% higher than that of presentcommercial titanium dioxide having the anatase crystal structure. Whenthe conversion to rutile has been less the advantage in hiding power andtinting strength is correspondingly less.

We claim:

1. In the preparation of a rutile seeding agent for the conversion of ahydrated titanium dioxide obtained by the hydrolysis of a titaniumsulfate solution to rutile, the steps which comprise precipitation ofhydrated titanium dioxide from a sulfate solution by admixture andheating with suflicient hot aqueous alkaline solution to produce afactor of acidity of 40%, whereby precipitation is completed in a fewminutes, converting the hydrated oxide to an alkali metal titanate,treating the titanate with an aqueous monobasic acid in sufficientamount to completely convert the alkali metal content to the salt of themonobasic acid and to theoretically convert 20-50% of the titaniumcontent to the salt of the monobasic acid, and boiling the resultantmixture for about 1 hour whereby the titanium content is substantiallyconverted to a rutile conversion seed capable of converting to rutile ahydrated titanium dioxide obtained by the hydrolysis of a titaniumsulfate solution upon admixture therewith in amounts of about l5% byweight and calcination at temperatures of about 900 C. to about 1000 C.

2. In the preparation of a rutile seeding agent for the conversion of ahydrated titanium dioxide obtained by the hydrolysis of a titaniumsulfate solution to rutile, the steps which comprise precipitation ofhydrated titanium dioxide from a sulfate solution by admixture andheating with sufiicient hot aqueous alkaline solution to produce afactor of acidity of 40%, whereby precipitation is completed in 'a fewminutes, converting the hydrated oxide to a sodium titanate, treatingthe titanate with hydrochloric acid in sufiicient amount to completelyconvert the alkali metal content to the salt of the monobasic acid andto theoretically convert ,50% of the titanium content to the chloride,and boiling the resultant suspension for about 1 hour whereby thetitanium content is substantially converted to a rutile conversion seedcapable of converting to rutile a hydrated titanium dioxide obtained bythe hydrolysis of a titanium sulfate solution upon admixture therewithin amounts of about 1-5% by weight and calcination at temperatures ofabout 900 C. to about 1000 C.

3. A method of preparing a rutile seeding agent having rutile crystalstructure which comprises the precipitation of hydrated titanium dioxidefrom a titanium sulfate solution by admixture and heating withsufiicient hot aqueous alkaline solution to produce a factor of acidityof 40%, whereby precipitation is completed in a few minutes, convertingthe hydrated oxide to an alkali metal titanate, water-washing thetitanate, treating the washed titanate with an aqueous monobasic acid insufficient amount to completely convert the alkali metal content to thesalt of the monobasic acid and to theoretically convert 20-50% of thetitanium content to the salt of the monobasic acid, and heating theresultant suspension and thereby obtaining a rutile seeding agent havingrutile structure capable of converting to rutile a hydrated titaniumdioxide obtained by the hydrolysis of a titanium sulfate solution uponadmixture therewith in amounts of about 1-5% by weight and calcinationat temperatures of about 900 C. to about 1000" C.

WINFRED J. CAUWENBERG. LEIF AAGAARD.

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